1. Field of the Invention
The present invention relates generally to coating compositions and, more particularly, to polyorganosiloxane compositions that can form mar resistant coatings and coatings made from these compositions.
2. Background of the Art
A variety of substrates, including those made of glass, plastic, metal, or masonry, are usefully coated with protective films to reduce their tendency to be abraded, to provide sacrificial surfaces, and to resist corrosion. It is generally desirable that protective coatings have good weathering and adhesion. It is also desirable that such coatings be resistant to thermal shock, mechanical shock, heat, humidity, and common chemicals. In addition, the coatings must be practical to prepare, apply, dry, and cure.
Some plastics are desirable substitutes for glass due to lighter weight, economically advantageous fabrications, and breakage resistance. However, commercially available plastic surfaces are less abrasion resistant, mar resistant, and scratch resistant than glass. Thus, protective coatings for plastic substrates are of particular interest. This is particularly true within the field of optical uses for plastic materials (e.g., as lenses, windows, covers, containers, ophthalmic layers, and the like) where damage to the surface of the polymer can significantly affect its function.
Much effort has been exerted in this field, and several different technical approaches have been described. In particular, work has been carried out on the development of polyorganosiloxanes crosslinked by the condensation of silanol groups.
Mayazumi, in U.S. Pat. No. 3,837,876, describes a polymer formed by reacting an aminosilane with an epoxysilane, dissolving the resulting product in a solvent, and then coating various substrates with the solution of the product. Ender, in U.S. Pat. No. 3,166,527, describes the mixing of an epoxysilane with an aminosilane, then coating surfaces with both the unreacted mixture and the reacted (partially polymerized) mixture. The coating was cured by standing at room temperature for a longer period of time or by heating for a shorter period of time.
Koda, in U.S. Pat. No. 3,961,977, describes the use of a partially hydrolyzed (10-40%) aminoalkoxysilane and an epoxysilane in a coating mixture. The two are dissolved in a solvent that may include a ketone. The ketone, although not claimed as a blocking agent to polymerization, appears to impede polymerization, thus extending the pot life.
Treadway and Carr, in U.S. Pat. No. 4,378,250, describe the use of aldehydes or ketones as blocking agents in polymeric compositions derived from certain aminosilanes and epoxysilanes. The reference also describes the nuance of increasing the hydrolysis of the silanes to above 40%. The reference describes greater abrasion resistance in the cured product and longer pot life in the curable composition because of the presence of the ketone acting to retard the reaction between the amine functionality and the epoxy functionality on the various reactants. The use of two different silanes make the formulation of the coating ponderous and, furthermore, there is a limited dye tintability range that can be obtained by varying the ratio of epoxy to amino within the bounds of compositions described for attaining the desired level of abrasion resistance. Replication of these compositions shows crosslink equivalent weights of at least about 173 when fully cured.